The invention concerns a method for operating a self-propelled battery-operated power tool, in particular a self-propelled lawnmower, comprising an electrical propelling drive by means of which the power tool is moved across a surface, wherein the surface is surrounded by a border delimitation wire through which electrical signals are sent and the electromagnetic field of a sent signal is received by at least one receiving coil within the power tool and induces a received signal, wherein the received signals of a receiving coil are evaluated in an evaluation unit of the power tool and the evaluation unit outputs an output signal to a control unit of the power tool which, as a function of the output signal, controls the travel direction of the power tool.
Self-propelled lawnmowers are known in general. EP 1 025 472 B1 discloses a system comprised of a battery-operated lawnmower with an electric propelling drive that, for determining the travel direction and the travel speed, is controlled by a control unit. The surface to be mowed by the lawnmower is defined by a border delimitation wire and two signals are to be applied simultaneously onto its wire loop. A first alternating voltage signal oscillates at a first frequency and a second alternating voltage signal oscillates at twice the frequency. In receiving coils within the lawnmower, the alternating electromagnetic fields of the permanently sent alternating voltage signals induce received signals that are evaluated by an evaluation unit in such a way that a conclusion can be made whether the power tool is located within or outside of the wire loop. When the first signal crosses the zero line, a reference point is set. At this point in time the system checks where the subsequent maximum of the second signal that is oscillating at twice the frequency is located. When it is on the plus side, the receiving coil of the robot mower is within the wire loop; when it is on the minus side, the receiving coil of the robot mower is outside of the wire loop. Based on this evaluation, the travel direction of the lawnmower is controlled.
EP 1 512 054 A1 discloses a control for a self-propelled battery-operated lawnmower wherein the field strength of the received electromagnetic field of signals sent through the wire loop is evaluated. The travel direction of the power tool is determined based on the strength of reception of the received signal.
A problem with known self-propelled lawnmowers is the occurrence of random external and internal electromagnetic field interferences that in practice may reach the magnitude of the received signal and can even be greater. Such interferences can be caused internally by the drive motors of the lawnmower or its motor power electronics or externally by external high-voltage lines, radio signals or even other neighboring robot mowers. When interferences occur, a precise determination of the location of the robot mower within or outside of the wire loop is made more difficult or even impossible.
It is an object of the invention to provide a method for operating a self-propelled battery-operated power tool that, with minimal technical expenditure, can operate even in areas of strong electromagnetic field interferences in a safe and failure-free way.